Sacrificial cathodic protection is a technique that is used to control the corrosion of steel. It involves connecting to the steel a base metal or alloy that is less noble than steel, such as a metal or alloy of zinc, aluminium or magnesium. The base metal is consumed by anodic dissolution and in the process current flows to the steel which becomes the protected cathode of the base metal-steel couple.
One application of sacrificial cathodic protection is to protect steel reinforcement in concrete. U.S. Pat. No. 6,022,469 describes the use of sacrificial anodes in the repair of corrosion damaged concrete structures. Sacrificial anode systems may be applied to a concrete surface (U.S. Pat. No. 5,650,060) or may be embedded as compact discrete anodes within the concrete (U.S. Pat. No. 6,572,760). A description of discrete impressed current and sacrificial anodes is given in U.S. Pat. No. 6,217,742. The ratio of anode to steel surface area affects the anode current density required to protect the steel and the relatively small surface of discrete anodes means that relatively high anode current densities may be required from discrete anodes. However they are strongly attached to the structure and anode attachment failures are much less common for discrete anodes than for the non-discrete anodes that are applied to concrete surfaces.
Sacrificial anodes sometimes experience reduced current output as a result of base metal inactivity. This was observed with the usual zinc and magnesium sacrificial anodes employed in soil environments. As a result, chemical backfills into which the zinc or magnesium is placed were developed. These backfills consist of a powdered mixture of hydrated gypsum, bentonite and sodium sulphate and the recipe will depend on the resistivity of the environment. In higher resistivity environments, the sodium sulphate content of the backfill is sometimes increased.
Sacrificial anodes have been applied to reinforced concrete exposed to marine environments where chloride contamination maintains base metal activity and anode current output. However the addition of halide ions like chloride to sacrificial anode assemblies is discouraged because these ions induce corrosion of the steel, and in other concrete environments a reduction in anode current output presents problems.
WO 2004/057057 describes the use of conventional gypsum, bentonite, sodium sulphate backfills to maintain the current output of sacrificial anode assemblies applied to a concrete surface. The advantage of using such backfills is that anode performance has been characterised. However, such backfills are associated with a number of disadvantages. They are in powdered form and are not easily retained on the concrete surface. It is difficult to prevent active components in the backfill from leaching out particularly when they are applied while the concrete or repair mortar is still fluid. The backfill tends to expand or contract with changing moisture conditions and this dimensional instability is difficult to accommodate in concrete and could result in a loss of contact between the anode and the surrounding concrete.
The use of sulphate or chloride ions in the backfill is discouraged by the deleterious effects these ions can have on the reinforcing steel and concrete. Alternative options devised to maintain base metal activity in concrete include exposure of the base metal to alkali and humectants (deliquescent materials). U.S. Pat. No. 6,303,017 describes the use of compounds like KOH and LiOH to maintain an environment around the base metal that is sufficiently alkaline to prevent base metal passivation. LiOH is preferred because KOH can induce a problem known as alkali silica reaction in concrete. However, LiOH does present health and safety problems that restrict its use. A combination of an alkali and a humectant such as LiNO3 are described in U.S. Pat. No. 6,572,760. U.S. Pat. No. 6,217,742 describes the use of combinations of the humectants, (LiNO3 and LiBr), to enhance the current output of sacrificial anode assemblies. However, while halide ions like bromide and chloride are very efficient in maintaining base metal activity, they can also induce corrosion of the steel reinforcement. To overcome such a corrosion risk, U.S. Pat. No. 6,793,800 describes a sacrificial anode assembly that uses a combination of a humectant and a steel corrosion inhibitor.
This invention is concerned with backfills that maintain base metal activity in concrete and improved methods of minimising the deleterious effects of the materials that maintain base metal activity on the surrounding reinforced concrete. This involves giving the backfill increased dimensional stability, containing the activating material of the backfill in the vicinity of the anode and providing a means of countering the deleterious nature of the adverse effects resulting from leaching of the activating material into the surrounding concrete.